Aircraft in the smaller weight classes are generally driven by propellers that are connected to internal combustion engines. The advantage of such a propulsion system is that the good thermal efficiency of the internal combustion engine in particular results in low consumption and low emissions, including in partial load and no-load operation, and the costs of the engine itself are low. The conversion of the mechanical power into thrust by the propeller in this case is efficient, particularly in the lower speed range, so that shorter take-off distances and favorable rates of climb of the aircraft are achieved. In contrast, in particular the following disadvantages are associated with such a drive:                The propeller has a large diameter, so that a high landing gear is necessary, which is difficult to retract and produces high drag. A high landing gear also increases the risk of overturning.        The propeller is usually disposed forward in the fuselage of the aircraft, so that the view and the design freedom of the aircraft are still very limited, resulting in almost all models looking the same.        Large centrifugal forces of the large propeller occur, so that there is also a reaction about the other two axes to control commands of the pilot about one of the three axes.        The propeller slipstream has a swirl that acts on the surfaces aft of the propeller and causes a rotation of the aircraft there together with the angular momentum of the slipstream, the rotation being dependent on the selected engine power. As a result, changing the engine power also causes an unintended and unwanted change of the trim position.        The propeller absorbs less power with increasing speed, so that either the engine power has to be throttled or a propeller adjustment is required in order to prevent overspeeding of the engine.        The inherent efficiency of the drive is affected negatively, because the air is accelerated on the nose of the fuselage and said increased flow speed determines the air resistance on the surface of the fuselage.        The propeller slipstream contains large turbulences that hinder the targeted laminarity of the flow over the involved areas in the slipstream.        The propulsion system is relatively heavy in relation to the delivered power.        
As an alternative to the propeller drive, jet and turbofan drives are used in more expensive aircraft, which at least partially avoid the mentioned disadvantages because the centrifugal and swirl forces are almost completely absent, the diameter of the drive being very much smaller, so that the design freedom of the design and the inherent efficiency can be significantly improved and all reactions to control or disturbance impulses act on each axis separately, being fully decoupled. The view from the cockpit is also less restricted and the height of the landing gear can be reduced. Such advantages are described as positive by the pilots of jets (positive jet characteristics).
The disadvantages of jet engines compared to propeller engines are that                the costs of the propulsion turbine are very much higher,        the thermal efficiency of the turbine is very much lower, especially in partial load and no-load operation,        the thrust in the lower speed range is lower and the noise emission of the turbine is comparatively very high.        
In DE 1 781 112 A it is therefore proposed that an internal combustion engine drives an encased propeller (ducted propeller or fan), so that good thermal efficiency is achieved with a thrust generator of relatively small diameter. In order to achieve good inherent efficiency and at the same time achieve the jet characteristics described, the ducted propeller is integrated within the fuselage in this case. Although the targeted advantages are achieved in this way, other significant disadvantages also occur. As a result of the integration within the fuselage, the inflow of air to the ducted propeller or fan plane is partly obstructed by elements of the structure, so that when the air enters the fan (ducted propeller) the flow speed is reduced in a few segments by so-called wake depressions in the speed profile. This phenomenon is also already known with pusher propellers. The consequence thereof is that the rotor blade encounters cyclically different incident flows during its rotation and thus the real angle of attack of the propeller blade varies and consequently the pressure on the blade also varies. Three negative effects are produced in this way:                The forces acting on the rotor blade change suddenly, so that the blade is stimulated to vibrate and thus material fatigue effects of the structure, the rotor blades and the drive shafts occur.        The flow conditions change from an optimal angle of attack to a degraded (possibly even detached) flow condition, so that a large additional load torque (=high engine power) is absorbed, without thereby contributing to thrust generation.        The cyclically occurring pressure steps are emitted as acoustic oscillation that act inwards and outwards as a disturbing noise and in addition are conspicuous with uncomfortably high frequencies for typical parameters (number of blades, revolution rate, number of noise sources).        